Firing process for forsterite ceramics



Feb. 13, 1962 w. J. KocH FIRING PROCESS FOR F'ORSTERITE CERAMICS Filed Deo. 7, 1959 vll?.

JNVENToR. william J.l Koch BY AGENT United States .3 020,619 FIRING PROCESS'FOR FORSTERITE CERAMICS ,William J. Koch, Livingston, NJ., assigner Vto VRadio lConiorationof America, a corporation of 'Delaware `Filed Dec. 7, 1959, Ser. `No.'-857,479 -4 Claims. (Cl. 25-157) 'This inventionrelates to ain-improved: process of making `ceramic'parts and, more particularly, to an improved tiring process y.for making'shaped "articles of'forsterite ceramic compositions. l

fCerarnic partsused in industries such as Ithe electron vvuibemanufacturing industryrmust-often meet rigid specifications with V'regard to properties such lias porosity,

A"flexural strength, `andfthermal expansion. 'Ihis is Vparticularly -true of ceramic parts intended =fori-use in cerl'tain vacuum'tubes having relatively small dimensions and v-very close dimensional -tolerances.

ldeveloped vacuum tube knownras the NuvistoriisaV disc having a:diameter'ofabout.0;400"i.002 anda height of about 0.065i.002". rl`he disc is pierced with 11 holes each having a diameter of about 0.0175" to 0.0180.

vThis disc must have vrela-tively'lowinternal porosity, high .flexural strength, and a-particular coeicient of thermal expansion matching :closely'that of the lmetal 'envelope towhich it is -to behermetically sealed. Since `this disc .must be joined Yto several metal parts, it isdesirable 'that it `be easily coated with a thin, adherentimetal coating. The metal coating must withstand high temperatures.

.Because of the coeiiicient of expansion required and the other ,desirable properties they musthave, these ceramic :.discs are Ypresently made of -forsterite ceramics having ythe major `crystalline phase ZMgOfSiOZ. :Since no na*- -ural mineral is known having'the exact composition of `forster'ite ceramics, the Amaterial is made synthetically. It nlayfbe` ma'de,'for example, :from a composition consisting of VMontanatalc (3MgO-4SiO2-H2O) 51.25%,ffused MgO 27.12%, Clinchiel'd feldspar (peta-sh feldspar),

7.81%, Kentucky `Special `ball clay `(Al'2O3-'SiO2-2H2O) 4.72%,andBaCO3'(precipitated) 9.10%.

The above ingredients are'blended'by ballimilling with an organic millingmedium as disclosed in vapplication Serial No. 857,478, led December 7, 1959, of Lawrence ^P. Garvey. After-blending and dispersing-the'particles as vdescribed in the above-mentioned applicationythe material is dried and Athen mixed with organic materials which serve asplasticizers,,binders-and lubricants, and

. again thoroughly blended.

-A suitable ingredient for'theforganic material portion is-polyethylenev glycol, one form' ofwhich is commercially available as CarboWaXAOOO. Another suitable ingredient forthe organicmaterial portion istrigamine stearato. 1A molding.compositionfis prepared by mixing :3000 g. ofthe dried material, bal1-milled as labove described, 120 g.

lCarbowax Y4000, l45 g. vtrrigamine stearato, and 900 cc. water.

This composition is mixed in va `Simpson mixer ffor about 40 minutes to make the mixture moldable. The'mixture is driedffor'Zhours at 75 `C.,'shredded in a vHobart"food'chopper, and-dried Iagain for about 1 hour at 75 C. The mixturel is then placed in a Stokes oscillating granulator andthe material'is forced through a 20 mesh screen. The material is then dried andplaced in a-gyrat'ory sifter vto select the fraction Whichpasses arent yfiring for 'l2-24 hours, -oi the organic binders and lubricants which are ainecesthrough a 40 mesh screen but isretainedgnfa 1`O0inefsh screen. The selected fraction isjdriedfovernighhatfZS 21C. The dried material, made as,,above-described,.is,'then compression-molded `into small discs. Y ,'Ifhe unre'd.discs have a diameter of 0.452 and a height of OLON-0.078.. Eleven holes, each having-a'tdiameterof'0.'.0205l are molded into the disc. The pressedweight ofeaeh disc is l.4478 gram and the pressed density is'2.29 gramsperfco The normal tiring schedule of fthese bodies vvaswair- Aireiiring -is A necessary f'to sburn sary part of the composition. 1 HoWevenit Vwas f found that air-tiring did not produce a-.completelvsatisfactory product using relatively shortiiring schedules- Using short tiring Schedules, .i.e., .under two hours,.forv;the iinal tiring, the internal porosity could inot vbe-made 10W enough to obtain a suiciently high density.l @onsequently, properties suchA as diexural Ystrength -were-otten not high enough and, because ofthe -.too ihigh porosity, metallzing usually could not be properly carried out so that the-metallic coatfwas suiliciently uniform. lI\`/I'o re over, under 'microscopic examination, the 'ffoisterite'crystals often did not appear to be'welliformed'somestreamers of glass could be seen in the ltired body, nthe body 'appeared to be laminated rather-thanhomogene'ous' throughout, and there werel relatively `large finternalpores. l

which-results in the uniformproduction of ceramic parts having very lowporosity, improved vilexural strength,"and

low thermal expansion. n

A further objectfoi the inven-tion isto provide "an iinproved tiring process 'for making forsterite' ceramics *in which the tiring time is greatly shortened compared tothe normal tiring times usually associated*1with-1the-=nranu facture of this type of ceramic. v

-A particular featureof lthe present invention is Ethe 'discovery that forsterite ceramic partshavingthe desinble properties mentioned `above can be made byf'rshrring Vthe yparts for Ia relatively shontiperiod oftirnefinir Vorother oxidizing atmosphere until loss onignitiondue to burning o of the organic lmatter ainddrivving fffof VWater ofcrystallization and other volatilesis complete, and then Vtiling for a relatively short periodof time in a non-oxidizing atmosphere un-til `the product is completely vitritied. The total time-elapsedfin the double-iringprocess is relatively short. i

The drawing l isA a` graphical representationl of a1-typical `example, of vtheV process of tiring foi-steritescompositions in accordance vwith the present invention as yLapplie'dJto themaking of small: ceramic discs. t

F1o. 1a .is anfing scheduieforthe nrstf'nnng in-sir. FIG. 1b Vis a firing .schedule .for fthe secoudfir'ing'in anon-oxidizing atmosphere.

. A specific ,example A'of lrthe ring process, Vas applied 'to the ceramic discs above-described, will-now begive'n.

The "furnace used was a laboratory tubular'type 1"hm/"ing insideldiameter of 2" and 'a length of`50". .t'rvvas heated by` globars. Provision wasma'deffor'introducing gas atmospheres Yas desired :and apparatus was also utilized -for ,pushing parts to `be manufactured through thefurnace at a desired rate of travel. Only thementer `5 'of the furnace'isheatedto maximum tempel ure.

For the'rst tiring, thepressed discs are placed. o'np alumina Setters. With .the 'furnace `brought up to,y ein- Patented Feb. 13, 1962 perature, and the atmosphere within the furnace being air, the following firing schedule was used.

Room temperature to 600 C.-7 minutes At 600 C.-10 minutes 600 C. to 1150 C.-8 minutes At 1150 C.-7 minutes 1150 C. to room temperature- 8 minutes This first firing in air serves to oxidize and burn-off all of the binders and lubricants and it also drives off all the water of crystallization and other voltatile ingredients.

These parts which have been given a first firing, as described above, are then given a second firing as follows in the same type of furnace with an atmosphere of 90% nitrogen-% hydrogen, by volume.

Room temperature to 300 C.-18 minutes 300 C. to 1l00 C.9 minutes 1100 C. to l375 C.-5 minutes At 1375 C.-61/2 minutes 1l00 C. to 300 C.-l3 minutes 300 C. to room temperature-8 minutes.

Using commercial production schedules, the above second firing schedule can be shortened considerably. For example, parts can be brought from room temperature to 1100" C. in about 9 minutes followed by the same maximum heat schedule and can be taken from l100 C. back down to room temperature in about 13 minutes.

The fired pieces have the following dimensions: diameter0.400 i .002"; height-0.066"i.002". The parts have a fired density of 2.94-3.02 grams per cc. The diameter of each of the 1l small holes is now .0175" to .0180".

The ceramic parts made by the process described above have all of the properties as to density, flexural strength, thermal conductivity, and thermal expansion required for use in the newly developed vacuum tubes. They also can be metallized uniformly so that metal parts can be brazed thereto. Compared to parts which had been previously made by air-firing alone, the parts made in accordance with the present invention have improved properties. Average modulus of rupture of parts made by air-firing only (parts having a firing schedule of 24 hours)22,00022,500 lbs. per square inch. Parts made as above described have an average modulus of rupture of 25,500 lbs. per square inch. Parts which were rapidly air-fired using a schedule similar to that given above for the improved process of the present invention never got below a diameter of 0.406 and most of them had diameters between 0.410" and 0.412. Parts made in accordance with the processes of the invention have average diameters of 0.400"i0.002". The reduction in diameter is a measure of the increased density and lower porosity of the improved product. Thermal expansion coefficient of the bodies between 25 C. and 925 C. is between 114 10'1 and 116)('10-7 in./in./ C. Under microscopic examination, parts made in accordance with the present invention showed well-formed crystals, little or no unreacted MgO, good homogeneity, no laminations and small internal pores. The reasons for the irnproved results obtained with the present process are not entirely understood. The improvements may be due to a lowering of surface tension of the forsterite and glass particles or to other causes. But, whatever the reasons, the improved firing process has resulted in uniform production of parts having the properties needed for their intended use.

Certain variations can be made in the improved process of the invention without departing from the scope thereof. Firing time of the first firing in air will depend considerably on the percentage of the binders and lubricants present as well as the sizes of the parts being made. Under any circumstances, it is simply necessary to fire long enough to drive o substantially all volatile substances and oxidize all organic mater. Moreover, the parts need not be cooled down to room temperature before going to the second firing step. After loss on ignition is complete, the parts may be advanced directly to the next firing step and brought to maximum firing temperature.

Attempts have been made to eliminate the first firing in air, without success. Without the first firing in air or other oxidizing atmosphere, the parts become blistered and bloated and the organic matter is converted to carbon which remains distributed throughout the part, changing its physical properties and electrical properties.

Certain variations can also be made in the second firing step. The atmosphere which was used in the above example for the second firing was nitrogen and 10% hydrogen. This is known as forming gas and is easily available in industrial operations at low cost. Other non-oxidizing atmospheres can be used, however. Parts were made equally well in vacuum, or in argon or helium or mixtures of the two, or in hydrogen containing sufficient water vapor to inhibit reduction of the ceramic composition to metal. Actually, somewhat better results were obtained using a wet hydrogen atmosphere but dry hydrogen caused some of the compounds to be reduced partially to the metallic state which is undesirable. The wet hydrogen atmosphere was obtained by bubbling hydrogen at the rate of 12 cubic feet per hour through water maintained at 26-32 C. The rate of bubbling was varied between 10-14 cubic feet per hour without noticing any substantial change in the fired products.

The rate at which the parts were brought up to maximum firing temperature in the second firing step was found to be significant within limits. The maximum rate of temperature rise possible was found to be about C. per minute and it was found preferable to maintain the bodies within the 1100 to S1375 C. range for at least 5 minutes so that the bodies would be fired uniformly.

The maximum firing temperature during the second tiring step can be varied somewhat but not a great deal unless other conditions are changed. Using alumina setters, when the maximum temperature was raised to 1400 C., small cracks formed throughout the pieces. These did not form at 1385 C. At 1365 C., the time required at the maximum temperature was about 1/2 hour and since short firing time is one of the objectives here, it is apparent that decrease in the maximum temperature below about 1375 C. is not desirable even though somewhat lower temperatures are operative. To achieve a desirably short firing time of `5-10 minutes at the maximum firing temperature, the maximum firing temperature should be about l3751390 C.

What is claimed is:

l. In a method of making ceramic parts of approximately forsterite composition, having relatively high density and high modulus of rupture, the process comprising preparing a blended and dried mixture of talc (SMgO-4SiO2-2H2O), fused MgO, potash feldspar, ball clay (Al2O3-Si02'2H2O) and precipitated BaCOa, mixing the blended mixture with organic plasticizers, binders and lubricants to prepare a molding composition, molding parts of said composition, subjecting said molded parts to a first firing in air to burn off substantially all organic matter and to drive off substantially all volatile matter, and then subjecting said parts to a second firing for a relatively brief period of time in a non-oxidizing atmosphere until said parts are completely vitried, said second firing including maintaining said parts at a maximum temperature of about 1375 to 1390 C. for about 5 to l0 minutes.

2. In a method of making ceramic parts of approximately forsterite composition, having relatively high density and high modulus of rupture, the process comprising preparing a blended and dried mixture of talc,

5 fused MgO, potash feldspar, ball clay and barium carbonate, mixing the blended mixture with organic plasticizers, binders and lubricants to prepare a molding composition, molding parts of said composition, subjecting said molded parts to a first firing in air to burn oif substantially all organic matter and to drive off substantially all volatile matter, and then subjecting said parts to a second firing for a relatively brief period of time in an atmos' phere consisting essentially of 90% nitrogen and 10% hydrogen by volume until said parts are completely vitrified, said second tiring including maintaining said parts at a maximum temperature of about 1375 to 1390 C. for about 5 to 10 minutes.

3. In a method of making ceramic parts of approximately forsterite composition, having relatively high density and high modulus of rupture, the process comprising preparing a blended and dried mixture of talc, fused magnesium oxide, potash feldspar, ball clay and precipitated barium carbonate, mixing the blended mixture with organic plasticizers, binders and lubricants to prepare a molding composition, molding parts of said composition, subjecting said molded parts to a rst tiring in air to burn ofi substantially all organic matter and to drive ott substantially all volatile matter, and then subjecting said parts to a second iin'ng in a Wet hydrogen atmosphere until said parts are completely vitrified, said second ring including maintaining said parts at a maximum temperature of about 1375 to 1390n C. for about 5 to 10 minutes.

density and high modulus of rupture, the process com-'ff prising preparing a blended and dried mixture of talc,

fused magnesium oxide, potash feldspar, ball clay and.

precipitated barium carbonate, mixing the blended mixture with organic plasticizers, binders and lubricants to prepare a molding composition, subjecting said molded parts to a rst firing in air to burn off substantially all organic matter and to drive 01T substantially all volatile matter and then subjecting said parts to a second tiring for a relatively brief period of time in a nonoxidizing atmosphere until said parts are completely vitrilied, said second tiring including maintaining said parts at a maximum temperature of about 1375 to 1390 C. for about 5 to l0 minutes, and raising and lowering the temperature up to the maximum and back down to room temperature at a rate not exceeding about 100 C. per minute.

References Cited in the tile of this patent UNITED STATES PATENTS 1,528,759 Fallon Mar. 10, 1925 2,369,266 Thurnauer Feb. 13, 1945 2,626,445 Albers-Schoenberg Jan. 27, 1953 2,912,340 Pincus Nov. 10, 1959 FOREIGN PATENTS 618,094 Germany Sept. 4, 1935 

1. IN A METHOD OF MAKING CERAMIC PARTS OF APPROXIMATELY FORSTERITE COMPOSITION, HAVING RELATIVELY HIGH DENSITY AND HIGH MODULUS OF RUPTURE, THE PROCESS COMPRISING PREPARING A BLENDED AND DRIED MIXTURE OF TALC (3MGO.4SIO2.2H2O), FUSED MGO, POTASH FELDSPAR, BALL CLAY (AL2O3.SIO2.2H2O) AND PRECIPITATED BACO3, MIXING THE BLENDED MIXTURE WITH ORGANIC PLASTICIZERS, BINDERS AND LUBRICANTS TO PREPARE A MOLDING COMPOSITION, MOLDING PARTS OF SAID COMPOSITION, SUBSTANTIALLY ALL VOLATILE MATTER, A FIRST FIRING IN AIR TO BURN OFF SUBSTANTIALLY AL ORGANIC MATTER AND TO DRIVE OFF SUBSTANTIALLY ALL VOLATILE MATTER, AND THEN SUBJECTING SAID PARTS TO A SECOND FIRING FOR A RELATIVELY BRIEF PERIOD OF TIME IN A NON-OXIDIZING ATMOSPHERE UNTIL SAID PARTS ARE COMPLETELY VITRIFIED, SAID SECOND FIRING INCLUDING MAINTAINING SAID PARTS AT A MAXIMUM TEMPERATURE OF ABOUT 1375* TO 1390*C. FOR ABOUT 5 TO 10 MINUTES. 